One of the important ways to increase mass resolving power and mass accuracy in mass analysis is to design a mass analyzer in which ions are measured during, or after, a long flight path. This kind of mass analyzer has recently been realized in two forms, the multi-turn ToF analyzer and the electrostatic or magnetic field ion trap analyser. In the multi-turn ToF analyzer, a reflecting field is generated by mirror electrodes so that a long, but folded flight path is achieved. A detector including a secondary electron multiplier is used, where, following the folded, long flight, ions splash onto the dynode of the detector and disappear, while an electric current signal is generated to give a ToF mass spectrum. In the electrostatic or magnetic field ion trap configuration, an ion's oscillatory motion induces image current in a pick-up electrode. The induced image current is continually recorded as the ion continually oscillates in the trapping field. The image current signal, after being amplified by a low noise amplifier, is converted to a frequency spectrum using Fourier transform and the frequency spectrum is then directly related to a mass spectrum of the trapped ions.
An early example of a high resolving power mass spectrometer is the so called FTICR, first disclosed in M. B. Comisarow and A. G. Marshall, Chem. Phys. Lett. 25, 282 (1974), where a superconducting coil is used to generate a high intensity, uniform magnetic field to trap ions. Because the coil is large and needs to be cooled to a very low temperature, this instrument is very expensive to build and difficult to run and maintain.
An electrostatic ion trap mass analyser is more attractive because it avoids use of a high strength, high stability superconducting magnet. The Orbitrap, disclosed in Anal. Chem., 2000, 72 (6), pp 1156-1162. by Alexander Makarov, is one example of electrostatic ion trap mass analysers where ions oscillate back and forth in the axial direction while, at the same time, rotating around a central, spindle-shaped electrode. To keep the axial oscillations harmonic, the central and outer electrodes of the Orbitrap need to be very accurately machined so as to achieve a so-called hyper-logarithmic potential inside the trap volume.
It is not necessary for the electrostatic ion trap mass analyser to have a field structure that allows ions to perform harmonic motion in a particular axial direction, such as in the Orbitrap. PCT Publication No WO 2012/116765 Li Ding et al. describes an electrostatic ion trap mass analyser including a first array of electrodes and second array of electrodes to create an electrostatic electric field in the space between the arrays. When both arrays are supplied with the same pattern of voltage, the resultant electric field causes ions to undergo periodic, oscillatory motion in the space between the electrode arrays, ions being repeatedly reflected isochronously in a flight direction and focused substantially at a centre plane, midway between said first and second arrays. An amplifier circuit is used to detect image current related to the mass-to-charge ratio of ions undergoing the periodic, oscillatory motion in the space between the first and second arrays of electrodes. A structure having multiple electrodes is advantageous because it is easier to tune by application of suitable voltages after the analyser has been manufactured. One of the disclosed embodiments (FIG. 9) in WO2012/116765 has a circular configuration, where the field-defining electrodes of each array include a circular, central electrode as well as a plurality of concentric, flat-surfaced ring electrodes, located radially outwards of the central electrode. The two arrays are arranged co-axially on the central axis of the analyser and ions are trapped close to the centre plane which is equidistant to the electrodes in the first and second arrays.
In the development of high resolving power ToF mass analyzers, many configurations of multi-turn ToF system have been designed. In US Publication No US 2010/0044558 A1, Sudakov disclosed a multiple-reflection time-of-flight device constructed using a pair of rectangularly-shaped planar electrode arrays. Ions are reflected in a flight direction (x) by two ion mirrors formed by parallel electrode strips of the planar arrays, and in a drift direction (z) by another reflection field formed by another set of electrode strips of the same planar arrays. Isochronous motion of ions of the same mass-to-charge ratio is achieved during each cycle in the (x-axis) flight direction, and for one reflection in the (z-axis) drift direction.
In U.S. Pat. No. 7,919,748 B2 by Curt Flory et al., another multiple-reflecting ToF system also includes a pair of planar electrode arrays, but these are circular in shape. Two sets of planar electrodes are disposed opposite one another, parallel to one another and axially offset from one another, the electrode structure generating a cylindrically symmetric, annular electric field surrounding a cylindrical, substantially field-free, central region, the electric field comprising an annular, axial focusing lens region and an annular mirror region surrounding the lens region.
These known multi-turn mass analysers have planar electrode arrays comprising multiple, flat electrodes mounted on a surface of an electrically insulating substrate in a close-packed configuration (e.g. the gap of electrodes is 2 mm in the multi-turn mass analyser disclosed in U.S. Pat. No. 7,919,748). Such electrode structures can be manufactured with relative ease because the electrodes can be formed on the substrate surface, in a desired pattern, by printing or by an alternative technique, such as cut-to-separate. However, in such a flat, packed electrode structure, the gaps between electrodes have to be narrow to avoid field distortion due to the effect of surface charge that accumulates on the substrate between electrodes. When ions undergoing oscillatory motion have energies of several keV, beam focusing (or similar measures designed to prevent beam divergence) necessitate provision of high voltage differences between neighbouring electrodes and, sometimes, such neighbouring electrodes are supplied with voltages of opposite polarity. According to examples in both WO 2012/116765 and U.S. Pat. No. 7,919,748, these voltage differences can exceed 10 kV, so there is potential for discharge and surface tracking. In U.S. Pat. No. 7,919,748 B2, Flory et al suggest depositing electrically resistive material in the gaps between electrodes. This might avoid the surface charge problem and might allow the gap between the adjacent electrodes to be increased to an extent. However, this approach requires that the resistivity of the electrically resistive material has an extremely high degree of homogeneity; otherwise, the electric field in the mass analysis space might be distorted. Moreover, when there is a high voltage difference between two electrodes, bridged by the resistive coating, current will pass through the resistive layer producing Joule heat. This causes the temperature to rise which, in turn, affects the stability of the high voltage supply and results in out-gassing in the mass analyzer, where ultra-high vacuum is usually necessary for long flight paths.
It is an object of this invention to provide a mass analyser which at least alleviates the afore-mentioned problems associated with known mass analysers.